Kinetics of d-lactic acid across muscle fiber membrane will be studied. The effects of concentration, temperature, and presence of 1-lactic acid on rate constant will be determined. Intracellular pH will be directly measured in barnacle muscle fibers by means of pH-sensitive electrodes. The values obtained under various extracellular conditions will be compared with steady state DMO distribution. Rate of change of pHi will be examined under the influence of intracellular injections of acids and bases.